Efficient shuffling of mobile drive units for inventory holders

ABSTRACT

An inventory system has mobile drive units that freely and independently move about a facility to transport inventory holders. The mobile drive units may operate through communications with other drive units, or under a more centralized control of a management module. For various operating scenarios, the mobile drive units are directed to shuffle the inventory holders in a manner that minimizes travel of the mobile drive units, thereby improving overall system efficiency. One or more single mobile drive units may be used to transport inventory holders to and from a region, and to sequentially reposition or slide each of the inventory holders within the region according to a priority ordering.

BACKGROUND

Modern inventory systems, such as those in mail-order warehouses, supplychain distribution centers, airport luggage systems, and custom-ordermanufacturing facilities, face significant challenges in responding torequests for inventory items. As inventory systems grow, the challengesof simultaneously completing a large number of packing, storing, andother inventory-related tasks become non-trivial. For instance, inproduct distribution centers (e.g., fulfillment centers), vastquantities of products are processed for shipment to consumerstraditionally using manual labor and/or mechanical handling equipment(e.g., conveyor systems, forklifts, etc.).

Inventory systems that are tasked with responding to large numbers ofdiverse inventory requests typically exhibit inefficient utilization ofsystem resources, including space, equipment, and manpower. This can, inturn, result in lower throughput, unacceptably long response times, anever-increasing backlog of unfinished tasks, and generally poor systemperformance. Additionally, expanding or reducing the size orcapabilities of many inventory systems requires significant changes toexisting infrastructure and equipment. As a result, the cost ofincremental changes to capacity or functionality may be prohibitivelyexpensive, thereby limiting the ability of the system to accommodatefluctuations in system throughput.

Accordingly, there remains a need for improved techniques for handlingproducts in a distribution center that reduce the potential for damagedproducts and increases efficiency in delivering products to a consumer,while minimizing operating costs.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is set forth with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical items or features.

FIG. 1 illustrates an inventory system having multiple regions andmobile drive units that carry inventory holders about the regions.

FIG. 2 shows a side view of a mobile drive unit and an inventory holderemployed in the inventory system.

FIG. 3 is a flow diagram illustrating an example shuffle process thatmay be implemented by the inventory system.

FIGS. 4A-4K present a sequence of illustrations to show use of theshuffle process of FIG. 3 to shuffle inventory holders at a pickstation.

FIGS. 5A-5F present a sequence of illustrations to show use of theshuffle process of FIG. 3 to shuffle inventory holders at a storageregion.

FIGS. 6A-6F present a sequence of illustrations to show use of theshuffle process of FIG. 3 to shuffle inventory holders at an inventorypier.

FIG. 7 is a block diagram of select components in a management moduleused in the inventory system of FIG. 1.

FIG. 8 is a block diagram of select components in a mobile drive unitused in the inventory system of FIG. 1.

DETAILED DESCRIPTION

Inventory systems are utilized by many entities for storing and managinginventory. For example, some retailers may utilize a warehouse of racksthat store inventory items in various bins. When an order for a specificinventory item needs to be filled by the retailer, a worker typicallyretrieves the inventory item from the bin where the inventory item isstored.

Inventory systems of the present disclosure utilize one or more mobiledrive units to automatically retrieve inventory holders from warehousestorage. The inventory holders may be entire racks of shelves, with eachshelf having bins for various inventory items. Mobile drive units may beself-powered robotic devices configured to move freely about thewarehouse. Racks may be transported by mobile drive units to a pickstation for performing inventory operations. The station may include ahuman operator and/or automated equipment to remove desired inventoryitems from the inventory holders and complete orders. In someoperations, the items are removed from inventory holders and placed onorder holders, which too can be maneuvered by the mobile drive units.Throughout this document, reference is made to inventory holders as theracks that are moved about by the mobile drive units. It is noted thatinventory holders is used in the general sense as structures that holdinventory items, items that are part of an order, packaging elements forthe orders, and essentially any other item or element that might be usedby the inventory and fulfillment system. As such, inventory holders mayalso be referred to as holders, order holders, container holders, bins,and so forth.

The ability for mobile drive units to transport inventory items to astation for performing inventory operations rather than or in additionto workers manually retrieving inventory items from stationary racks mayvastly increase efficiency and productivity of the inventory system. Forexample, by utilizing mobile drive units, an inventory system may becapable of fulfilling more orders per hour than previous solutions.Mobile drive units may move about the warehouse and/or retrieveinventory holders in response to commands and/or instructions receivedfrom an automated guidance system. For example, a management module maycontrol administration and coordination of certain elements and/or taskswithin the warehouse. The management module may receive orders forinventory items in the warehouse and coordinate task assignments forfulfilling the orders, including providing instructions to mobile driveunits to transport racks with requested inventory items to an inventorystation for completion of an order. The management module may alsoprovide guidance at the inventory station for how the order is to beassembled.

To use a simple illustration, in a distribution warehouse for an officesupply store, the management module may receive an order for a printer,several ink cartridges, and a box of paper. The management module maydetermine the locations of these items within the racks of storage inthe distribution warehouse. The items may, for instance, be on theshelves of three separate racks. The management module may issue tasksfor three different mobile drive units to retrieve each rackrespectively holding the printer, ink cartridges, and paper, and for themobile drive units to transport the racks holding those items to aninventory station where the three items may be picked from the racks andpacked into a box for shipment. In doing so, the management module maycoordinate the various tasks such that all of the orders received by themanagement module are fulfilled in an efficient manner.

Orchestrating movement of the mobile drive units is non-trivial. This isparticularly true at the pick station where human and/or mechanicalpickers retrieve items from the inventory holders. The mobile driveunits are continuously moving the inventory holders into and out fromthe pick stations to place the appropriate items near the pickers at theright time. For existing pick stations, current inventory systems face achallenge of increasing the lines per hour (lph) so that pickers mayretrieve more items per hour, thereby improving efficiency andthroughput of the inventory system.

The inventory systems described herein utilize efficient shufflingtechniques in which the mobile drive units reposition the inventoryholders in ways that present appropriate items at the pick station atthe correct time, while minimizing the amount of movement each mobiledrive unit makes to reposition the inventory holders. The efficientshuffling techniques may be used at various locations or stations of theinventory system, including at pick stations, deep storage locations,and high volume inventory piers. One example shuffling techniqueinvolves defining multiple locations at a pick station for arranginginventory holders in priority order. Such locations may include aprimary set of pick locations in which the inventory holders arepositioned next to the picker and a secondary set of staging locationsadjacent to, but spaced from, the pick locations. One or more mobiledrive units may then be used to efficiently deliver inventory holdersfrom storage to the staging locations, remove the recently-pickedinventory holders from the pick locations to the staging locations priorto returning them to storage, and sequentially move up each of theinventory holders in the pick locations in priority order. In somecases, a single mobile drive unity may perform the shuffling techniques.In other cases, one or more mobile units are used to deliver inventoryunits to the pick station and remove the inventory units after itemshave been picked, and one mobile unit is used to reshuffle the inventoryholders among the pick locations at the pick station.

For instance, suppose a pick station has three pick locations to holdthree inventory holders. Each pick location has an area footprint toaccommodate an inventory holder at rest within the pick location. Thefirst pick location is closest to the picker and a first inventoryholder holding the current items to be picked by the picker is placed inthe first pick location. The second pick location is next to the firstpick location to hold a second or on-deck inventory holder holding thenext items to be picked by the picker after the items on the firstinventory holder have been picked. The third pick location is next tothe second pick location to hold a third inventory holder. Furthermore,suppose there are one or more temporary or staging locations near thepick locations.

According to one shuffle implementation, when the picker has finishedwith the first order or inventory holder, a mobile drive unitrepositions the first inventory holder from the first location to astaging location. Thereafter, the same mobile drive unit repositions thesecond inventory holder from the second pick location to the first picklocation vacated by the first inventory holder. Subsequently, the samemobile drive unit repositions the third inventory holder from the thirdpick location to the second pick location vacated by the secondinventory holder. This sequential sliding of the inventory holders keepsitems readily available to the picker, while minimizing the amount ofwasteful travel time by the mobile drive units. Further, the same mobiledrive unit or other drive units may be used to deliver new inventoryholders to the pick station, initially depositing them at staginglocations, and removing the recently-picked inventory holders (such asthe first inventory holder noted above) from the staging locations backto storage or other regions of the warehouse.

This and other examples of operating an inventory system are describedbelow. The techniques discussed herein may be implemented in manydifferent ways, by many different systems. Various representativeimplementations are provided below with reference to the figures.

FIG. 1 shows one illustrative example of an inventory system 100 thatmay be used to implement a technique for efficient shuffling ofinventory holders. The inventory system 100 may be arranged in afacility or warehouse (e.g., distribution facility, fulfillment center,etc.) that is logically organized into areas or regions associated withvarious functions. In the illustrated example, the warehouse includes astorage region 102, a pick station 104, and an inventory pier 106. Inpractice, depending upon the size of the inventory system 100, thewarehouse may hold more than one of the storage regions 102, pickstations 104, and inventory piers 106, or the warehouse may beconfigured without the storage region 102, or the pick station 104, orthe inventory pier 106.

The inventory system 100 includes a management module 110, multiplemobile drive units 112, and inventory holders 114. Only some of theinventory holders 114 are shown referenced with the number 114 for easeof illustration. The mobile drive units 112 are independent,self-powered robotic devices that may move freely about the warehouse,under their own direction or through coordination by the managementmodule 110. In alternative implementations, the mobile drive units 112may be configured to move inventory holders 114 along tracks, rails,cables, or other guidance elements traversing the associated workspace.In such implementations, mobile drive units 112 may receive powerthrough a connection to the guidance elements, such as a powered rail.In some embodiments, the inventory system 100 may include two or moredifferent types of mobile drive units 112 having various capabilitiesand specifications. Moreover, although illustrated and discussed asthough mobile drive units 112 are a particular type of mobile driveunit, mobile drive units 112 may refer to various types of mobile driveunits. For example, one type of mobile drive unit 112 may be operable totransport relatively large, heavy, or bulky inventory items. Anothertype of mobile drive unit 112 may be operable to transport relativelylighter and/or more compact inventory items. Mobile drive units 112 mayaccordingly have various power trains, load capacities, and otherappropriate specifications to transport particular inventory items invarious types of the inventory holders 114 within the inventory system100.

The mobile drive units 112 may be used at various times to transport theinventory holders 114 around the warehouse among the regions. Forinstance, the mobile drive units 112 may transport the inventory holders114 between the storage region 102 and the pick station 104 or inventorypier 106.

Each inventory holder 114 may be implemented as a physical structure tohold various inventory items. The inventory holder 114 has a physicallength, width, and height that may be standardized or varied within theinventory system. As used herein, the inventory holders 114 may beconfigured to hold essentially any type or size of item or be used forany number of purposes, including, but not limited to, carrying pallets,storing shipping supplies, holding garbage, supporting empty boxeswaiting for inventory, supporting filled boxes with items once ordersare fulfilled, and so on. Furthermore, as used herein, inventory holdersalso include holders for other types of products or items and henceinclude order holders.

In one implementation, the inventory holder 114 may be formed as a rackhaving multiple shelves to support various types of inventory items. Forinstance, the inventory holders 114 may include multiple storage binswith each storage bin capable of holding a different type of inventoryitem. The inventory holders 114 are capable of being carried, rolled, orotherwise moved by the mobile drive units 112. Each inventory holder 114may have a plurality of faces, and each bin may be accessible throughspecific faces. The rack is free-standing when at rest, but can belifted and moved by the mobile drive units 112. The mobile drive units112 may be configured to rotate inventory holders 114 at appropriatetimes to present particular faces of inventory holders 114 and theassociated bins to an operator or other components of inventory system10. One example is described below in more detail with reference to FIG.2.

The mobile drive units 112 transport the inventory holders 114 betweenany number of predefined physical locations on the warehouse floor. Forillustration purposes, rectangular areas are depicted in FIG. 1 todesignate physical locations within the facility that may be used toplace an associated inventory holder. Each location may accommodate aninventory holder 114. That is, each inventory holder 114 may stand atrest within the area of the floor reserved or otherwise predefined as astorage location 114. The storage region 102, for example, has multiplestorage locations 120 arranged in sets of nine storage locations withaisles between the sets. An inventory holder 114 may be placed within acorresponding storage location 120 until needed by a picker at the pickstation 104 or inventory pier 106. In other layouts, there may be moreor less than nine storage locations per zone in the storage region 102.

One or more mobile drive units 112 are provided in the storage region102 to shuffle inventory holders 114 among the storage locations 120 andto transport the inventory holders between the storage region and otherregions in the warehouse. Two loaded drive units 112(1) and 112(2) areshown in the storage area 102 carrying associated inventory holders114(1) and 114(2), respectively, down aisles between the sets ofpredefined storage locations 120. An unloaded drive unit 112(3) is alsoshown moving through an aisle between the predefined storage locations120.

The pick station region 104 is designed with multiple locations 122 and124 to accommodate associated resting inventory holder 114. In FIG. 1,the pick station 104 has five locations arranged in two linear rows. Afirst line of three pick locations 122 is mapped next to a picking areain which a picker 126 picks inventory from the inventory holders 114 atthe pick locations 122 and loads them into boxes or containers supportedby another inventory holder 114(4) mounted on a mobile drive unit112(4). In this illustration, the picker 126 is a human, although thepicker may alternatively or additionally involve a mechanical pickingapparatus. A second line of two staging locations 124 is mapped adjacentto, but spaced from, the first line of pick locations 122. The staginglocations 124 temporarily hold inventory holders 114 on the way to andfrom the pick locations 122 of the pick station 104. It is noted thatfive locations are merely representative, and that pick stations 104 maybe designed with more or fewer than five locations.

A mobile drive unit 112(5) is shown at the pick station 104. The mobiledrive unit 112(5) may be essentially dedicated to the pick station toshuffle inventory holders 114 among the pick and staging locations 122and 124 of the pick station 104 according to the techniques describedherein. Alternatively, the mobile drive unit 112(5) may be directed tothe pick station 104 on an as-needed basis. In some implementations,multiple drive units 112 may be used to service the pick station 104,with one or more mobile drive units used to carry inventory holders 114to and from the staging locations 124 and one or more mobile drive unitsto shuffle the inventory holders 114 among the pick and staginglocations 122 and 124 of the pick station 104.

The inventory pier 106 facilitates higher volume inventory. In manyinventory systems, a small percentage of high velocity and/or highvolume inventory are responsible for a large percentage of overallsystem activity. High velocity inventory may refer to popular inventorythat is requested by a large percentage of orders, while high volumeinventory may refer to inventory that is requested in large volumes byorders. Volume may refer to the physical amount of cubic space certaininventory items require in the warehouse. High volume inventory itemsmay refer to large or bulky items and/or items that are purchased inlarge quantities by individual orders. In some inventory systems, forexample, the top 1% of inventory items may account for over 20% ofoverall system activity. Highly popular items may require acorrespondingly high use of mobile drive units to transport inventoryholders to inventory stations. Similarly, high volume inventory mayrequire relatively more labor to replenish inventory holders storingsuch items such that breaking down such items into inventory holders maybe less efficient than merely using the inventory for orders in themanner it arrives from a wholesaler. A more efficient method of fillingorders for such items may be realized using an inventory pier. Theinventory pier may store highly popular and/or high volume inventory.Inventory holders that store popular or high volume inventory may bepositioned along one side of the inventory pier. The inventory pier mayaccordingly be configured to include a configurable number of fixedpositions for top inventory items. For an example inventory systemstoring 3,000 unique inventory items, as few as thirty unique inventoryitems may account for a high percentage of system activity due to highvelocity or high volume of such items. Thus, in such an inventorysystem, the inventory pier may include a sufficient amount of definedareas to store the 30 top inventory items at the inventory pier.

Order holders may, for example, be racks of shelving that are configuredto hold boxes or containers that will be used to fill orders forinventory items. Each box or container may be associated with aparticular order. For instance, an order holder may store a cardboardbox that is to be used for shipping an order for several ink cartridgesand several reams of paper. Order holders with orders needing such itemsmay visit positions along the inventory pier corresponding to inventoryholders storing inventory items needed by the orders. Operators mayremove items from the inventory holders and place them into appropriateboxes or containers in the order holder. If necessary, the order holdermay then be taken to an inventory station to fill items remaining on theorders in the boxes or containers of the order holder.

To illustrate, in the office supply example discussed above, paper maybe a high volume inventory item and ink cartridges may be a highlypopular item. Accordingly, a pallet of paper and an inventory holderstoring various ink cartridges may be stationed at the inventory pier.An order for several packages of paper and an ink cartridge may befilled by moving an order holder a location along the inventory pieropposite to the pallet storing the paper where an operator may transferthe paper to the order holder. The order holder may then move to alocation opposite the inventory holder storing the ink cartridges wherethe same or a different operator may transfer the ink cartridge to theorder holder. If the requested printer is not already stored at theinventory pier, a mobile drive unit may transport the order holder to aninventory station, where another mobile drive unit may transport aninventory holder containing the printer so that the order can be filled.

In FIG. 1, the inventory pier 106 is shown with multiple designated pierlocations 130 to accommodate the inventory holders 114. Five pierlocations 130 are illustrated to accommodate five corresponding holders114, although there may be more or fewer pier locations per inventorypier 106. One mobile drive unit 112(6) is shown at the inventory pier106 to shuffle inventory holders 114 among the pier locations 130according to the techniques described herein. Two loaded mobile driveunits 112(7) and 112(8) are shown at rest next to a pier 132 and holdingassociated inventory holders 114(7) and 114(8) (or more specifically inthis case, order holders) for access by a picker 134 (again, representedas a human although a mechanical device may be used). In this example,the inventory holders 114(7) and 114(7) may be holding packages orcontainers for order fulfillment, where the picker 134 removes selecteditems from the inventory holders 114 positioned in pier locations 130and loads the items into order containers on the inventory holders114(7) and 114(8).

In some implementations, multiple mobile drive units may be used tobring inventory holders to and from the pier locations 130 of theinventory pier 106. Incoming inventory holders may be placed intemporary locations while a single mobile drive unit, such as unit112(6), removes an existing inventory holder that has been recentlypicked by the picker 134 from a pier location 130 and replaces it with anew inventory holder with new items of interest for the picker 134.

In one implementation, the management module 110 orchestrates movementof the mobile drive units 112, directing them to various regions withinthe warehouse. The management module 110 coordinates transport of thevarious inventory holders among the regions in the warehouse.Furthermore, the management module 110 may be used to instruct themobile drive units to perform the shuffle processes within a particularregion (e.g., storage region 102, pick station 104, inventory pier 106,etc.). Generally, the shuffle process involves directing a mobile driveunit 112 to reposition a first inventory holder from its currentlocation within the region to a temporary location within the regionthat is adjacent or proximal to the current location. The mobile driveunit 112 leaves the first inventory holder in the temporary location andsubsequently positions a second inventory holder into the locationvacated by the first inventory holder. The mobile drive unit 112 thenlifts the first inventory unit to transport the first inventory holderaway.

The management module 110 may use any form of communication to directthe mobile drive units. In one implementation, the management module 110and the mobile drive units are configured to communicate using wirelesstechnologies, such as a wireless local area network (WLAN). As oneexample, some embodiments of mobile drive unit 112 may communicate withmanagement module 110 and/or with one another using Wi-Fi (IEEE 802.11),Bluetooth (IEEE 802.15), Infrared Data Association standards, or anyother appropriate wireless communication protocol. As another example,in a tracked inventory system 100, tracks or other guidance element uponwhich mobile drive units 112 move may be wired to facilitatecommunication between mobile drive units 112 and the management module110 and/or other components of inventory system 100.

In addition to directing the mobile drive units, the management module110 may receive and/or generate requests to initiate any of a number ofparticular operations involving the mobile drive units 112, inventoryholders 114, or other elements of inventory system 100. The managementmodule 110 may select components of inventory system 100 to performvarious operations and communicate commands, instructions, and/or otherappropriate information to the selected components to facilitatecompletion of these operations. The management module 110 may receiveorders for various inventory items and coordinate and administer variousappropriate tasks to fill the orders. For example, an order may specifyparticular inventory items that have been purchased by a customer andthat are to be retrieved from inventory system 10 for shipment to thecustomer. The management module 110 may receive the orders from anyappropriate system and generates task assignments based, in part, on theorders including requests for inventory items. Based on the orders, themanagement module 110 may identify particular operations to be completedinvolving inventory items stored or to be stored within inventory system100.

After generating one or more task assignments, the management module 110selects appropriate components to complete particular tasks andtransmits task assignments to selected components, such as the mobiledrive units, to trigger completion of the relevant tasks. The relevantcomponents then execute their assigned tasks. Each task assignmentdefines one or more tasks to be completed by a particular component.These tasks may relate to the retrieval, storage, replenishment, andcounting of inventory items and/or the management of mobile drive units112, inventory holders 114, or other components of inventory system 100.Depending on the component and the task to be completed, a particulartask assignment may identify locations, components, and/or actionsassociated with the corresponding task and/or any other appropriateinformation to be used by the relevant component in completing theassigned task.

While the appropriate components of inventory system 100 completeassigned tasks, the management module 110 may interact with the relevantcomponents to ensure the efficient use of space, equipment, manpower,and other resources available to inventory system 100. For example, themanagement module 110 may receive information from the variouscomponents of system 100 regarding their current location, state, and/orother characteristics. Based on its knowledge of the location, currentstate, and/or other characteristics of the various components ofinventory system 100 and an awareness of tasks currently beingcompleted, management module 100 can generate tasks, allot usage ofsystem resources, and otherwise direct the completion of tasks by theindividual components in a manner that optimizes operation from asystem-wide perspective. Moreover, by relying on a combination of bothcentralized, system-wide management and localized, component-specificdecision-making, particular embodiments of inventory system 100 may beable to support a number of techniques for efficiently executing variousaspects of the operation of inventory system 100.

As a result, the management module 110 may enhance the efficiency ofinventory system 100 and/or provide other operational benefits. Forexample, the management module 110 may determine particular inventoryholders 114 to be stationed at the pick station 104 or inventory pier106 based on determining that when placed at these locations, overallsystem activity may be reduced and/or picker efficiency is improved.Similarly, the management module 110 may group orders and assign tasksto fulfill orders such that order fulfillment activities are optimized.

FIG. 2 illustrates a side-view illustration of a mobile drive unit 112and inventory holders 114 employed in the inventory system 100. In thisillustration, one inventory holder 114(1) is at rest in a first locationnext to a human picker 202. The inventory holder 114(1) has multipleshelves that support a variety of items 204. The inventory holder 114(1)has open side faces to facilitate stocking of the items 204 onto theshelves and picking of items from the shelves.

The inventory items 204 represent any objects suitable for storage,retrieval, and/or processing in an automated inventory system 100. Forexample, a given inventory item may represent a single stock-keepingunit (SKU) of a unique inventory item. Thus, inventory items may referto a particular item and/or may refer to a quantity of items having aparticular SKU as appropriate. As one example, the inventory system 100may represent a retail distribution warehouse that stores bulk inventoryitems 204 for retail facilities, such as grocery stores and/ormerchandise warehouses. As another example, the inventory system 100 mayrepresent an e-commerce warehouse facility, and inventory items 204 mayrepresent merchandise stored in the warehouse facility.

A mobile drive unit 112 is also shown moving a second inventory holder114(2) into a second location next to the first location. The mobiledrive unit 112 is positioning a second inventory holder 114(2) into alocation for easy access by the picker 202 (such as at the inventorypier 102) or to place it in an on-deck location to be shuffled to thefirst location after the picker has completed picking items 204 from thefirst inventory holder 114(1) (such as at the pick station 104).

The mobile drive units 112 may transport the inventory holders 114 bydocking with and/or coupling to a particular holder and moving theholder while docked. For example, as illustrated, the mobile drive unit112 transports inventory holder 114 by moving beneath the inventoryholder 112 and lifting a docking head 206 that interfaces with a bottomsurface (or other portion) of inventory holder 114. The docking head 206may be controlled by an actuator of mobile drive unit 112 operable tolift inventory holder 114 when docked. The docking head 206 may couplethe mobile drive unit 112 to the inventory holder 114 or otherwisesupport the inventory holder 114 during transportation. The docking head206 may also include any appropriate features to facilitate coupling tothe inventory holder 114. For example, in some embodiments, ahigh-friction element may form all or a portion of the docking head 206,which abuts a portion of the inventory holder 114 while docked. In suchembodiments, frictional forces created between the high-friction elementof the docking head 206 and a surface of the inventory holder 114 mayinduce translational and rotational movement when docked.

The mobile drive unit 112 may utilize the docking head 206 to maneuverthe inventory holder 114, such as by lifting, rotating, and/or movingthe inventory holder 114 in any appropriate manner. In someimplementations, the mobile drive unit 112 may be capable of rotatingits docking head to rotate the inventory holder 114 while moving and/orwhile stopped. In addition or in the alternative, the mobile drive unit112 may be capable of rotating the docking head independently or as apart of the movement of the mobile drive unit 112 as a whole. Forexample, the mobile drive unit 112 may rotate the inventory holder 114as the mobile drive unit 112 executes a turn such that the inventoryholder 114 continues to face the original direction of movement.

The mobile drive unit 112 is further equipped with multiple drive wheels208 and multiple stabilizer wheels 210. A motor internal of a body 212is used to power the drive wheels 208, while the stabilizer wheels 210provide stability to the drive unit 112 to maintain balance of thecoupled inventory holder 114. Using the drive wheels 208, the mobiledrive unit 112 may transport the inventory holder 114 while the dockinghead is lifting inventory holder 112 or otherwise move the mobile driveunit 112 when the inventory holder 114 is undocked.

It should be noted that while a particular method of docking with theinventory holder 114 is illustrated, the mobile drive unit 112 may dockwith the inventory holder 114 by connecting to, lifting, and/orotherwise interacting with the inventory holder 114 in any othersuitable manner so that, when docked, the mobile drive unit 112 arecoupled to and/or support inventory holder 30 and can move inventoryholder 30. Further, in some embodiments, potentially some of the mobiledrive units 112 may not dock with the inventory holders 114 with theinventory holder 114, but rather may remain continually mated with aparticular inventory holder 114.

In some implementations, the mobile drive unit 112 may perform a shuffleprocess to efficiently place the next inventory holder 114 adjacent tothe picker 202. The mobile drive unit 112 may shuffle the inventoryholders according to a pattern programmed into the unit, and/or underthe direction of the management module 110. At the most basic level, themobile drive unit 112 moves a new inventory holder into the locationcurrently occupied by an existing inventory holder.

With reference to FIG. 2, suppose the mobile drive unit 112 is tasked toshuffle the second inventory holder 114(2) into the first locationcurrently occupied by the first inventory holder 114(1) adjacent to thepicker 202. This situation may represent, for example, a pick stationscenario where the picker 202 has completed picking items 204 from thefirst inventory holder 114(1) and is ready to pick items from the secondinventory holder 114(2). The mobile drive unit 112 initially movesbeneath the first inventory holder 114(1), mounts the docking head 206,and lifts the inventory holder off of the ground. The mobile drive unit112 then moves the first inventory holder 114(1) out of its currentlocation next to the picker 202 to a temporary adjacent location. Afterthe mobile drive unit 112 sets down the first inventory holder 114(1) atthe temporary location, the drive unit moves beneath the secondinventory holder 114(2). The mobile drive unit 112 mounts the dockinghead 206 and lifts the inventory holder off of the ground. The mobiledrive unit 112 repositions the second inventory holder 114(2) from itscurrent location to the location next to the picker 202 that was justvacated by the first inventory holder 114(1). The mobile drive unit 112sets down the second inventory holder 114(2) at the first location forthe picker to begin picking items 204. Meanwhile, the same or differentmobile drive unit 112 returns to the first inventory holder 114(1) atthe temporary location, lifts it up, and moves it to another region,such as to the storage region 102.

FIG. 3 shows this basic shuffle process 300 that may be implemented bythe inventory system 100. The process is illustrated as a collection ofblocks in a logical flow graph. Some of the blocks represent actionstaken by the mobile drive unit under the direction ofcomputer-implemented operations performed at the drive unit or themanagement module. In the context of software-based operations, theblocks represent computer-executable instructions stored on one or morecomputer-readable storage media that, when executed by one or moreprocessors, direct the mobile drive units to perform the recited acts.The order in which the operations are described is not intended to beconstrued as a limitation, and any number of the described blocks can becombined in any order or in parallel to implement the processes. It isunderstood that the following processes may be implemented with otherarchitectures than the inventory system 100 described above.

Adjacent to the blocks, the process 300 includes a pictorialrepresentation of a single mobile drive unit 112 performing the steps ofthe shuffle process with two inventory holders 114(1) and 114(2)according to the arrangement of FIG. 2.

At 302, a mobile drive unit 112 is moved to a first location at which afirst inventory holder 114(1) is positioned. The mobile drive unit 112is positioned under the inventory holder 114(1) and lifts it off of theground.

At 304, the mobile drive unit 112 repositions the first inventory holder114(1) from the first location to a temporary location proximal to thefirst location. As illustrated, the mobile drive unit 112 mayessentially move the inventory holder out of the first location and setit down.

At 306, the mobile drive unit 112 repositions the second inventoryholder 114(2) from its current or second location to the first locationvacated by the repositioning of the first inventory holder 114(1). Themobile drive unit 112 is positioned under the second inventory holder114(2), lifts it off of the ground, moves it to the first location, andsets it down again.

At 308, the mobile drive unit 112 (or another mobile drive unit)transports the first inventory holder 114(1) away from the temporarylocation.

The shuffle process 300 may be executed in any number of situations,such as shuffling inventory holders up the pick line at the pick station104, shuffling inventory holders within a set of storage locations inthe storage region 102, or shuffling the inventory holders at theinventory pier 106. The shuffle process involves minimum movement of themobile drive unit 112 to improve efficiency of the inventory system 100.Each of these scenarios is described below in more detail with referenceto FIGS. 4-6.

FIGS. 4A-4J show use of the shuffle process in a scenario 400 to shuffleinventory holders at a pick station 104. The pick station 104 has fivepredefined locations including a primary set of three pick locations 402and a secondary set of two staging locations 404. Three inventoryholders A, B, and C are positioned in the three pick locations 122(1),122(2), and 122(3), respectively, and a fourth inventory holder D ispositioned a staging location 124(2). It is noted that more or lesslocations and inventory holders may be arranged at a pick station 104.

One or more mobile drive units 112 may be used to shuffle the inventoryholders among the multiple pick locations and the multiple staginglocations at the pick station. For discussion purposes, this scenario isdescribed as being performed by a single drive unit. However, in otherimplementations, one mobile drive unit may be used to move the inventoryholders among the pick locations 122 and one or more other drive unitsare used to bring inventory holders to, and remove them from, thestaging locations 124.

In FIG. 4A, a mobile drive unit 112 retrieves a new inventory holder Dfrom a place separate from the pick station 104, such as the storageregion 102. The mobile drive unit 112 deposits the new inventory holderD in the staging location 124(2). In FIG. 4B, the mobile drive unit 112moves to a first pick location 122(1) adjacent to the picker. The mobiledrive unit 112 moves under the inventory holder A and engages theinventory holder A. This inventory holder A has just recently beencompleted by the operator, and is ready to be removed for restocking iffully picked, or maneuvered to another location (e.g., storage, pickstation, pier, shipping station, etc.) for subsequent usage. In FIG. 4C,the mobile drive unit 112 repositions the most recently-picked inventoryholder A from the first pick location 122(1) to an empty staginglocation 124(1).

Thereafter, in FIG. 4D, the mobile drive unit 112 moves to the secondpick location 122(2). In FIG. 4E, the mobile drive unit 112 repositionsan on-deck inventory holder B from the second pick location 122(2) tothe first pick location 122(2) vacated by the repositioning of therecently-picked inventory holder. Accordingly, the inventory holder B isnow positioned for the picker to more quickly pick items.

In FIG. 4F, the mobile drive unit 112 moves from the first pick location122(1) to the third pick location 122(3). In FIG. 4G, the mobile driveunit 112 repositions the inventory holder C from the third pick location122(3) to the second pick location 122(2) vacated by the repositioningof the inventory holder B. In this manner, the inventory holder C ismoved to the on-deck location as next up for the picker.

In FIG. 4H, the mobile drive unit 112 moves from the second picklocation 122(2) to the second staging location 124(2) to grab the newinventory holder D. In FIG. 4I, the mobile drive unit 112 repositionsthe newest inventory holder D from the staging location 124(2) to thethird pick location 122(3) vacated by the repositioning of the inventoryholder C.

In FIG. 4J, the mobile drive unit 112 moves from the third pick location122(3) to the staging location 124(1) to pick up the recently-pickedinventory holder A. In FIG. 4K, the mobile drive unit transfers therecently-picked inventory holder A away from the staging location 124(1)to remove the inventory holder A from the pick station 104.

In this scenario, the shuffle process enables the drive unit toefficiently slide a set of inventory holders up in priority at a pickstation. That is, the inventory holders at the pick locations 122 form aqueue where the inventory holder closest to the picker at pick location122(1) is highest priority. The picker is most likely to pick inventoryfrom this holder or put inventory into this holder, although the pickermay also pick inventory from other holders in the various picklocations. During shuffle, the next inventory holders B, C, and D aremoved up in the priority queue established by the pick locations.Additionally, a new inventory holder is delivered to the pick stationwhile the inventory holder most recently picked may be removed from thepick station.

It is noted that in some situations, the new inventory holder D may beplaced higher in the queue than other holders. For instance, themanagement module 110 may determine that the inventory holder D nowholds inventory items that the picker should work on next and hencereprioritize the existing queue. In this situation, the shuffle processrepositions the new inventory holder D from the staging location 124(2)to a higher priority pick location, such as locations 122(1) or 122(2)rather than the last available slot of location 122(3), that is ahead ofat least one other inventory holder already residing in the picklocation queue.

FIGS. 5A-5F show use of the shuffle process in a scenario 500 to shuffleinventory holders at a storage region 102. The storage region includesmultiple sets of storage locations 120, such as the set of ninelocations. Nine holders are positioned in the storage locations,including an inventory holder A in position 120(1) and an inventoryholder B in position 120(2). It is noted that more or less locations andinventory holders may be arranged within a predefined set of the storageregion 102.

In FIG. 5A, the mobile drive unit 112 moves into the first storagelocation 122(2), lifts the inventory holder B, and temporarily moves itto a temporary location 502. As shown in FIG. 5B, the mobile drive unitthen moves back through the storage location 120(2) to the internalstorage location 120(1) beneath inventory holder A.

In FIG. 5C, the mobile drive unit 112 lifts the inventory holder A, andmoves it to another temporary location 504. This leaves open two storagelocations 120(1) and 120(2). In FIG. 5D, the mobile drive unit 112 movesfrom the temporary location for the inventory holder A back to thetemporary location of the inventory holder B.

In FIG. 5E, the mobile drive unit 112 repositions the inventory holder Bfrom the temporary location 502 to the storage location 120(1) justvacated by the first inventory holder A. The mobile drive unit 112leaves the inventory holder B in the storage location 120(1) (or eventstorage location 120(2) and then returns to the temporary storagelocation 504 where the inventory holder A is temporarily resting. InFIG. 5F, the mobile drive unit removes the inventory holder A from thestorage region 102.

Accordingly, in this scenario, the shuffle process enables the mobiledrive unit to efficiently remove and replace inventory holders in astorage region. The travel distance of the mobile drive unit isminimized in this process, thereby improving efficiency of the system100.

FIGS. 6A-6F show use of the shuffle process in a scenario 600 to shuffleinventory holders at an inventory pier 106. The inventory pier definesmultiple pier locations 130, such as the five pier locations shown inthese figures. Five inventory holders are positioned in the pierlocations, including an inventory holder A in position 130(1). It isnoted that more or less locations and inventory holders may be arrangedin an area associated with the inventory pier 106. In this scenario 600,the mobile drive unit is removing an inventory holder A from which thepicker has retrieved the necessary items to fill an order and replacingit with a new inventory holder B with items that the picker is likely toneed in the near future to fill a new order.

In FIG. 6A, the mobile drive unit 112 brings a new inventory holder B tothe inventory pier. The mobile drive unit 112 temporarily places theinventory holder B in a temporary location near the permanent locations,proximal to the pier location 130(1). In FIG. 6B, the mobile drive unit112 moves to the pier location 130(1) to dock with the inventory holderA.

In FIG. 6C, the mobile drive unit 112 lifts the inventory holder A andrepositions it from the pier location 130(1) to a temporary locationbeside the pier locations. In FIG. 6D, the mobile drive unit 112 movesfrom the inventory holder A to the inventory holder B at rest in atemporary location.

In FIG. 6E, the mobile drive unit 112 lifts the inventory holder B andrepositions it into the pier location 130(1) just vacated by theinventory holder A. The mobile drive unit 112 undocks with the inventoryholder B, and then moves back to the inventory holder A in its temporarylocation. In FIG. 6F, the mobile drive unit 112 transports the inventoryholder A away from the inventory pier. For instance, the mobile driveunit transfers the inventory holder A to a storage location in thestorage area.

FIGS. 4-6 are merely representative examples of the shuffle processbeing used at various regions within an inventory system. These are notintended to be limiting, and other uses and variations of the processmay be used.

FIG. 7 shows select components in the management module 110 according toone possible implementation. The management module 110 may beimplemented by one or more computers having processing, memory, andcommunications capabilities. The module 110 may be a dedicated device,or a general computer system programmed to perform the managementoperations.

In FIG. 7, the management module 110 includes one or more processors 702and memory 704. The processor(s) 702 are configured to executeinstructions, such as those instructions stored in memory 704, or inother memory accessible to the processor(s) 702, such as storage incloud-based resources. The memory 704 may include computer-readablestorage media (“CRSM”). The CRSM may be any available physical mediaaccessible by a computing device to implement the instructions storedthereon. CRSM may include, but is not limited to, random access memory(“RAM”), read-only memory (“ROM”), electrically erasable programmableread-only memory (“EEPROM”), flash memory or other memory technology,compact disk read-only memory (“CD-ROM”), digital versatile disks(“DVD”) or other optical disk storage, magnetic cassettes, magnetictape, magnetic disk storage or other magnetic storage devices, or anyother medium which can be used to store the desired information andwhich can be accessed by a computing device.

Several modules, such as instructions, datastores, and so forth may bestored within the memory 704 and configured to execute on a processor(s)702. An operating system module 706 is configured to manage hardware andservices within and coupled to the management module 110 for the benefitof other components. A shuffle module 708 is configured to direct themobile drive units 112 to perform the shuffle process described hereinat various times and locations throughout the inventory system. Theshuffle module 708 may provide a list of tasks to fully perform theshuffle process, or may provide instructions as-needed for the mobiledrive unit to shuffle the inventory holders. In that way, the managementmodule 110 may either micro manage each movement of the mobile driveunits throughout a shuffle process, or may simply provide a plan andtiming for the mobile drive units to carry out on their ownindependently of further instructions. In this latter scenario, eachmobile drive unit 112 may be equipped with hardware and/or softwarecomponents that are configured to enable the unit to perform the shuffletechnique independently of the management module 110, as will bedescribed in more detail with reference to FIG. 8.

The management module 110 further includes a communication unit 710 tocommunicate with the mobile drive units or with other computing devices.The communication unit 710 enables access to one or more types ofnetwork, including wired and wireless networks. More generally, thecoupling between the management module 110 and any components in theinventory system may be via wired technologies, wireless technologies(e.g., RF, cellular, satellite, Bluetooth, etc.), or other connectiontechnologies. When implemented as a wireless unit, the communicationunit 710 uses an antenna 712 to send and receive wireless signals.

FIG. 8 shows select computing components that may be implemented in themobile drive unit 112. The computing components may include a controlunit 802 that is formed using one or more of generally computinghardware, specially-dedicated hardware, software, or firmware. Thecontrol unit 802 may execute shuffle code 804 that enables the mobiledrive unit 112 to perform the shuffle process described herein. Theshuffle code may be independently executed by the mobile drive unit 112,or alternatively through interaction with instructions received from themanagement module 110.

The mobile drive unit 112 may further be equipped with a communicationunit 806 to communicate with other mobile drive units and/or with themanagement module 110. The communication unit 806 enables access to oneor more types of network, including wired and wireless networks. Whenimplemented as a wireless unit, the communication unit 806 uses anantenna 808 to send and receive wireless signals.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as exemplary forms ofimplementing the claims

What is claimed is:
 1. A method comprising: positioning multipleinventory holders at corresponding multiple locations at a pick stationof an inventory system, the multiple locations including a primary setof multiple pick locations and a secondary set of multiple staginglocations; shuffling the inventory holders among the multiple picklocations and the multiple staging locations at the pick station usingone or more mobile drive units, the shuffling comprising: retrieving anew inventory holder and depositing the new inventory holder in a firststaging location of the multiple staging locations; repositioning arecently-completed inventory holder from a first pick location of themultiple pick locations to a second staging location of the multiplestaging locations; repositioning a next inventory holder from a secondpick location of the multiple pick locations to the first pick locationvacated by the repositioning of the recently-completed inventory holder;repositioning the new inventory holder from the first staging locationof the multiple staging locations to the second pick location of themultiple pick locations; and transferring the recently-completedinventory holder away from the second staging location to remove therecently-completed inventory holder from the pick station.
 2. The methodas recited in claim 1, wherein the shuffling is performed by a singlemobile drive unit.
 3. The method as recited in claim 1, wherein theshuffling is performed by multiple mobile drive units such that a firstmobile drive unit performs at least one of the retrieving of the newinventory holder or the transferring of the recently-completed inventoryholder and a second mobile drive unit performs the repositioning of theinventory holders among the multiple pick locations.
 4. The method asrecited in claim 1, wherein the multiple pick locations are prioritizedsuch that the first pick location is proximally closest to a picker andthe second pick location is adjacent to the first pick location.
 5. Themethod as recited in claim 1, wherein one or more additional inventoryholders reside at one or more other pick locations of the multiple picklocations, wherein the shuffling further comprises repositioning eachadditional inventory holder from a current pick location to an adjacentpick location to sequentially move the one or more additional inventoryholders closer to the first pick location, leaving a last pick locationvacant, and repositioning the new inventory holder from the firststaging location of the multiple staging locations to the last picklocation of the multiple pick locations.
 6. The method as recited inclaim 1, wherein one or more additional inventory holders reside at oneor more other pick locations of the multiple pick locations to form aqueue, wherein the shuffling further comprises repositioning the newinventory holder from the first staging location of the multiple staginglocations to a pick location of the multiple pick locations that isahead of at least one other inventory holder already residing in a picklocation so that the new inventory holder is higher in the queue thansaid at least one other inventory holder.
 7. A method comprising:retrieving a new inventory holder from a storage location andtemporarily placing the new inventory holder in a temporary staginglocation; moving a mobile drive unit to a first pick location at which afirst inventory holder is positioned; repositioning, using the mobiledrive unit, the first inventory holder from the first pick location tothe temporary staging location proximal to the first pick location uponthe first inventory holder being recently completed; repositioning,using the mobile drive unit, a second inventory holder from a secondpick location to the first pick location vacated by the repositioning ofthe first inventory holder, the second pick location being locatedadjacent to the first pick location; transporting, using the mobiledrive unit or another mobile drive unit, the first inventory holder awayfrom the temporary staging location; and repositioning, using the mobiledrive unit, the new inventory holder from the temporary staging locationto one of the second pick location or a third pick location, wherein thetemporary staging location and the first, second and third picklocations are part of a pick station within a warehouse.
 8. The methodas recited in claim 7, wherein the first inventory holder isrepositioned to the temporary staging location for a temporary period oftime such that the transporting of the first inventory holder away fromthe temporary staging location is initiated no later than uponcompletion of the positioning of the second inventory holder into thefirst pick location.
 9. The method as recited in claim 7, furthercomprising repositioning, using the mobile drive unit, a third inventoryholder to the second pick location vacated by the repositioning of thesecond inventory holder.
 10. The method as recited in claim 7, furthercomprising: repositioning, using the mobile drive unit, a thirdinventory holder from a third pick location to the second pick locationvacated by the repositioning of the second inventory holder; andsubsequently repositioning, using the mobile drive unit, the newinventory holder from the temporary staging location to the third picklocation vacated by the third inventory holder.
 11. The method asrecited in claim 7, further comprising initially repositioning, prior tothe moving of the mobile drive unit to the first pick location, thesecond inventory holder from an original location to a temporarylocation to clear a path to the first pick location for the mobile driveunit.
 12. The method as recited in claim 7, further comprising initiallyretrieving the second inventory holder and temporarily positioning thesecond inventory holder in a staging location while the mobile driveunit repositions the first inventory holder from the first pick locationto the temporary staging location.
 13. An inventory system comprising: aplurality of mobile drive units that are remotely controllable totransport inventory holders about a warehouse; and a management moduleconfigured to direct movement of the plurality of mobile drive units totransport the inventory holders among regions in the warehouse, themanagement module being further configured to direct a mobile drive unitfrom the plurality of mobile drive units to perform a shuffle processwithin a region of the warehouse as follows: directing the mobile driveunit or another mobile drive unit from the plurality of mobile driveunits to retrieve a new inventory holder from a storage location andtemporarily placing the new inventory holder in a temporary staginglocation; directing the mobile drive unit to reposition a firstinventory holder from a first pick location within the region to thetemporary staging location within the region that is proximal to thefirst pick location upon the first inventory holder attaining acompletion status; directing the mobile drive unit to subsequentlyposition a second inventory holder from a second pick location into thefirst pick location vacated by repositioning the first inventory holder,the second pick location being adjacent to the first pick location;directing the mobile drive unit or another mobile drive unit from theplurality of mobile drive units to transport the first inventory holderaway from the temporary staging location within the region of thewarehouse; and directing the mobile drive unit or another mobile driveunit from the plurality of mobile drive units to reposition the newinventory holder from the temporary staging location to the second picklocation or a third pick location, wherein the region of the warehouseincludes a pick station.
 14. The inventory system as recited in claim13, wherein the management module is further configured to direct themobile drive unit to initially place the second inventory holder in atemporary location while the mobile drive unit subsequently repositionsthe first inventory holder from the first pick location to the temporarystaging location.
 15. The inventory system as recited in claim 13,wherein the management module is further configured to direct the mobiledrive unit to initially move the second inventory holder from anoriginal location to a temporary location to clear a path to the firstpick location for the mobile drive unit.
 16. The inventory system asrecited in claim 13, wherein the management module is further configuredto direct the mobile drive unit or said another mobile drive unit toinitially retrieve the second inventory holder from another region ofthe warehouse and temporarily position the second inventory holder in atemporary location while the mobile drive unit repositions the firstinventory holder from the first pick location to the temporary staginglocation.
 17. An inventory system comprising: a plurality of inventoryholders to store items, the plurality of inventory holders including atleast first, second, and third inventory holders wherein the firstinventory holder is positioned at a first pick location of a pickstation, the second inventory holder is positioned at a second picklocation of the pick station, and the third inventory holder ispositioned at a third pick location of the pick station; and a mobiledrive unit configured to shuffle the first, second, and third inventoryholders as follows: retrieving a new inventory holder and depositing thenew inventory holder in a first holding location; moving the firstinventory holder from the first pick location to a second holdinglocation upon the first inventory holder attaining a complete status,the second holding location being separate from the first, second, andthird pick locations, the first and second holding locations beinglocated adjacent to each other and separate from the first, second, andthird pick locations; subsequently moving the second inventory holderfrom the second pick location into the first pick location vacated bythe first inventory holder; and subsequently moving the third inventoryholder from the third pick location into the second pick locationvacated by the second inventory holder; and subsequently moving thefirst inventory holder away from the second holding location to removethe first inventory holder from the pick station, wherein the first andsecond holding locations are located within the pick station.
 18. Theinventory system as recited in claim 17, wherein the mobile drive unitis further configured to subsequently move a fourth inventory holderfrom the plurality of inventory holders into the third pick locationvacated by the third inventory holder.
 19. The inventory system asrecited in claim 17, wherein the mobile drive unit is a first mobiledrive unit and further comprising: a second mobile drive unit configuredto relieve the first mobile drive unit from bringing the new inventoryholder to the pick station and moving the first inventory holder fromthe second holding location to another region separate from the pickstation.
 20. The inventory system as recited in claim 19, furthercomprising a management module to send instructions to the first andsecond mobile drive units to coordinate movement of the first and secondmobile drive units.